IEEE Network - March / April 2017 - page 74

IEEE Network • March/April 2017
0890-8044/17/$25.00 © 2017 IEEE
Driven by growing spectrum shortage, LTE in
unlicensed spectrum (LTE-U) has recently been
proposed as a new paradigm to deliver better
performance and experience for mobile users
by extending the LTE protocol to unlicensed
spectrum. In this article, we first present a com-
prehensive overview of the LTE-U technology,
and discuss the practical challenges it faces. We
summarize the existing LTE-U operation modes
and analyze several means for LTE-U coexistence
with Wi-Fi medium access control protocols.
We further propose a novel hyper access point
(HAP) that integrates the functionalities of LTE
small cell base station and commercial Wi-Fi AP
for deployment by cellular network operators.
Our proposed LTE-U access embedding within
the Wi-Fi protocol is non-disruptive to unlicensed
Wi-Fi nodes and demonstrates performance bene-
fits as a seamless and novel LTE and Wi-Fi coexis-
tence technology in unlicensed band. We provide
results to demonstrate the performance advantag-
es of this novel LTE-U proposal.
The rapid growth of mobile wireless applications
and consumers continues to strain the limited
cellular network capacity and has motivated the
exploration of next generation fifth generation
(5G) wireless networks. The mobile industry
predicts a 1000
data traffic growth by 2020.
To meet this anticipated data growth demand,
both industry and academia are exploring various
advanced solutions to boosting network capacity
while continually providing high-level user experi-
ence to wireless customers. One natural direction
is developing new technologies to improve the
efficiency of limited licensed spectrum. Such pop-
ular advances include small cell base station (SBS)
coverage, massive multiple-input multiple-out-
put links, and device-to-device communications.
Despite these exciting solutions, one key obsta-
cle to network capacity expansion still lies in the
scarce licensed spectrum resources.
To overcome spectrum shortage, LTE in unli-
censed spectrum (LTE-U) technology has been
proposed and is currently under consideration by
the Third Generation Partnership Project (3GPP)
in its future standards. LTE-U technology allows
users to access both licensed and unlicensed
spectra under a unified LTE network infrastruc-
ture. It can provide better link performance, medi-
um access control (MAC), mobility management,
and larger coverage than simple Wi-Fi offloading
[1, 2].
Despite the many advantages of LTE-U, it still
faces certain technical challenges. One primary
issue is the coexistence with the incumbent Wi-Fi
systems. On one hand, LTE systems are specifi-
cally designed to operate in the licensed spec-
trum under the centralized control of network
units based on non-contention MAC protocols to
prevent packet collision among subscribers. On
the other hand, Wi-Fi users rely on carrier sense
multiple access with collision avoidance (CSMA/
CA) to reduce packet collision and use a con-
tention-based MAC protocol, that is, distributed
coordination function (DCF), to resolve package
collision through a random backoff mechanism.
Therefore, how to ensure a fair and harmoni-
ous coexistence environment for both networks
becomes a major challenge for LTE-U.
First, for the two conflicting MAC technolo-
gies to coexist in the same unlicensed band, their
mutual impact and interference must be carefully
controlled and managed. It is important that:
• Wi-Fi transmissions will not frequently collide
with LTE-U transmissions.
• LTE-U transmissions will not lead to a sub-
stantial drop of Wi-Fi throughput.
Second, we should require a coexistence propos-
al of LTE-U and Wi-Fi so as not to demand funda-
mental changes to the existing protocols defined
in LTE and Wi-Fi standards. This requirement is
essential to avoid any impractical retrofitting of
IEEE 802.11-compliant Wi-Fi radios or the rede-
sign of existing 3GPP standards.
To overcome such challenges, we first propose
a new wireless access point (AP) architecture,
hyper-AP (HAP), which is deployed by cellular
operators and functions simultaneously as an
LTE SBS and a Wi-Fi AP. By integrating both LTE
and Wi-Fi functions within the same HAP node, it
can then jointly coordinate the spectrum alloca-
tion and interference management for Wi-Fi and
LTE-U. Based on the use of HAP, we develop a
novel LTE-U and Wi-Fi coexistence mechanism
by embedding LTE-U signalling within the exist-
ing Wi-Fi protocol for seamless integration. Spe-
cifically, HAP will provide LTE user access in the
contention-free period (CFP) under a centralized
coordinator, whereas HAP continues to provide
normal Wi-Fi user access through its contention
period (CP) using the traditional CSMA/CA mech-
anism. The main merit of the newly proposed
HAP is that it can well support the standalone
mode for LTE-U transmission, which is found to
be a formidable hurdle in many other coexistence
In what follows, we first provide an LTE-U
technology overview, including operation modes,
coexistence technologies for LTE and Wi-Fi, and
current considerations of combining LTE-U tech-
Embedding LTE-U within Wi-Fi Bands for Spectrum Efficiency Improvement
Qimei Chen, Guanding Yu, Hesham M. Elmaghraby, Jyri Hämäläinen, and Zhi Ding
Qimei Chen is with Zhejiang
Guanding Yu (correspond-
ing author) is with Zhejiang
University and Southeast
Hesham M. Elmaghraby and
Zhi Ding are with the
University of California, Davis.
Jyri Hämäläinen is with Aalto
Digital Object Identifier:
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